1. Field of the Invention
This invention relates to thermally regenerated, reciprocating, internal combustion engines that employ a movable regenerator.
2. Description of Related Art
Thermal regeneration is the capturing of waste heat from a thermodynamic cycle (or a heat engine operating on some thermodynamic cycle), and the utilization of that energy within the cycle or engine to improve the cycle or engine's performance. This is commonly done with many heat engines, including Stirling engines, gas turbines, and Rankine cycle devices. In a gas turbine, consisting of a compressor, combustor, and turbine, the temperature of the air leaving the turbine is often greater than the temperature of the air leaving the compressor. If the energy in the turbines exhaust can be transferred to the air leaving the compressor, it will not be necessary to add as much heat (fuel) in the combustor to raise the air temperature to the desired turbine inlet temperature. This means that the same work is accomplished, but less fuel is employed. Therefore, the specific fuel consumption of such a thermally regenerated gas turbine is improved. Thermal regeneration of gas turbines is commonly accomplished by the use of a heat exchanger that transfers energy from the exhaust gases to the compressed air.
Gasoline and diesel engine operation is generally approximated by a thermodynamic cycle referred to as the Otto cycle. In principle, an Otto cycle can also be thermally regenerated. This would be done by transferring heat from the gases at the conclusion of the expansion stroke to the gases of the next cycle at the conclusion of the compression stroke. The benefits that can be thus attained are substantial. Fuel consumption is reduced in a manner similar to that of the regenerated gas turbine. In addition, a regenerated Otto cycle is thermodynamically capable of providing higher gas temperatures during the cycle, which results in even greater improvements in efficiency and power. Since reciprocating engines only experience these higher temperatures for brief times, they can withstand these higher temperatures to some extent. Thus the benefits of regeneration are even greater for an Otto cycle device than they are for the temperature limited gas turbine.
The advantages of thermally regenerated gasoline or diesel engines are readily apparent and quite substantial. Unfortunately, viable and effective means by which this can be accomplished have not previously been developed. The difficulty lies in the fact that the compression, heating and expansion processes occur in the same location--i.e. within the cylinder. This makes it difficult to conceive of some means by which the heat can be captured and transferred to the compressed air at a different time in the cycle. For a gas turbine, which is a steady flow device with the cycle processes separated in space, it is relatively easy to add a heat exchanger at the appropriate place. It is much more difficult to do this in a non-steady flow, reciprocating engine where all the processes occur in the same location.
The approach taken by most inventors who attempted to incorporate regeneration into reciprocating internal combustion engines was to separate the engine processes in space. In this way it becomes relatively easy to insert a heat exchanger between the engine components that accomplish each process. This has led to a number of approaches such as those of Hirsch (1874, U.S. Pat. No. 155,087), Martinka (1937, U.S. Pat. No. 2,239,922), Pattas (1973, U.S. Pat. No. 3,777,718), Bland (1975, U.S. Pat. No. 3,871,179), Pfefferle (1975, U.S. Pat. No. 3,923,011), Cowans (1977, U.S. Pat. No. 4,004,421), Stockton (1978, U.S. Pat. No. 4,074,533), Webber (1986, U.S. Pat. No. 4,630,447), Ruiz (SAE paper 930063, 1993), and Carmichael (Chrjapin Master's thesis, MIT, 1975). All of these approaches involve at least two cylinders, generally one in which compression occurs and a second where the combustion and expansion occur. In the flow passage connecting these cylinders or in one of the cylinders there is a stationary permeable material that comprises the regenerator. The regenerator is an alternating flow heat exchanger. The expanded combustion gases are passed through the regenerator and transfer thermal energy to it. During the next cycle compressed air is forced through the regenerator and absorbs this energy. Thus heat is transferred from the hot exhaust gases to the compressed air--the essence of thermal regeneration.
Unfortunately, none of these earlier approaches for regeneration of internal combustion reciprocation engines have been successful. The reason for their failure lies in a basic feature of those approaches--the separation of the processes into different cylinders. Because some air and exhaust is always trapped in the transfer passages, because of "blowdown" losses between cylinders in some designs, and because not all of the air can be regeneratively heated or cooled, or be in the appropriate locations at the optimum times, the performance of these engines is reduced.
More recently, a new approach has been conceived. This new approach allows the processes to occur within a single cylinder. The most unique feature of this new approach is a movable regenerator. This regenerator is in the form of a thin disc with a diameter essentially equal to the engine bore. This regenerator disc is located between the cylinder head and the piston. This moving regenerator sweeps through all of the internal volume of the cylinder twice during each engine operating cycle. As it moves through the gas in the cylinder, it exchanges energy with that gas. One sweep removes energy from the expanded combustion products. The other sweep transfers this energy to the compressed working fluid near the conclusion of the next compression stroke. The regenerator movement that occurs near the end of the expansion stroke and cools the combustion products is referred to as the regenerative cooling stroke. The regenerator movement that starts near the end of the compression stroke and heats the compressed air is referred to as the regenerative heating stroke. Inventions based upon this approach of a movable regenerator are included in the patents of Ferrenberg (1988, U.S. Pat. No. 4,790,284 and 1990, U.S. Pat. No. 4,928,658 ) and Millman (1981, U.S. Pat. No. 4,280,468).
Regenerated engines employing movable regenerators that sweep through the interior volume of the cylinder can be divided into two classes: those in which the combustion occurs between the piston and the regenerator (hot piston designs) and those in which the combustion occurs between the regenerator and the cylinder head (cool or cold piston designs). The "hot volume" is always the volume where the combustion occurs and the "cold volume" lies on the other side of the regenerator. The side of the regenerator that faces the hot volume is referred to as the hot side of the regenerator and the side of the regenerator that faces the cold volume is the cold side of the regenerator.
In addition to other regenerated engine inventions unrelated to this patent application, Millman (U.S. Pat. No. 4,280,468) discloses and claims a hot piston regenerated engine operating on a four stroke cycle. This engine of Millman's lacks a regenerative cooling stroke. Instead, he maintains the regenerator stationary and adjacent to the valves in the cylinder head while the blow-down and the exhaust occur. This is a serious deficiency in the manner in which the energy is extracted from the working fluid by the regenerator that can substantially degrade engine performance.
The previous disclosures of Ferrenberg (U.S. Pat. Nos. 4,790,284 and 4,928,658) cover both two and four stroke, and hot and cool piston regenerated engines. However, these earlier inventions have some basic deficiencies that are corrected by the substantially different operation of the regenerated engine disclosed herein. One such deficiency is that the regenerator remains stationary during the blowdown or fails to substantially complete the regenerative cooling stroke prior to the blowdown. This is similar to the deficiency of Millman's engine. Also, the earlier disclosures and claims of Ferrenberg specifically close the exhaust valve prior to the opening of the intake valve in all regenerated engines employing valves. It is highly advantageous to have both valves open at the same time, for a short period. Finally, these earlier regenerated engine inventions started the cooling stroke too late during the cycle.
Other substantial differences exist between the earlier inventions of Millman and Ferrenberg, and the regenerated engine disclosed herein. All of these are discussed in greater detail in the section entitled "Detailed Description of the Invention".